1. Fields of the Invention
The present invention relates to a monitoring system for riverbed elevation monitoring at bridge piers, especially to a telescopic probe monitoring system for riverbed elevation monitoring at bridge piers in real time manner.
2. Descriptions of Related Art
In recent years, natural disasters caused by storms and associated floods become more frequent and severe all over the world. When the typhoon or hurricane comes, it causes serious flooding in the rivers and severe scour around the bridge pier leading to the exposure of bridge foundations. As a result, the lateral load acted on the bridge pier is insufficient and the bridge may be tilted or broken. Once the flooding is too violent, the pier may even be washed away, and the bridge collapses and people passing through the bridge may fall into the river. The above situation not only results in traffic interruption, but also great damages to human lives, common property and local construction. Now in order to observe the riverbed scour at bridge piers and make an alarm, a monitoring system is developed. Thus the severe condition of bridge pier scour can be found as early as possible so as to assure the bridge safety and provide a precaution alarm. The traffic safety is ensured and the secondary damage caused by the broken bridge can be avoided. Moreover, the lifetime of the bridge is also extended by strengthening and repair taken in time. Furthermore, by long-term record of monitor data, a complete database of the bridge pier scour is established for bridge pier protection and riverbed restoration in the future.
Among a lot of technologies that monitor the bridge pier scour available in the literatures, the most common methods are as followings:
Ground penetrating radar: is a non-destructive method that uses electromagnetic radiation in the high frequency band of the radio spectrum, and detects the reflected signals from subsurface structures. Thus the elevation changes of the riverbed under the water are obtained. The advantage of this method is that the changes in the riverbed elevation can be recorded continuously yet the shortcoming is that the operation is time-consuming and staffs require professional training.
Numbered bricks: a brick array formed by bricks with fixed size and sequential numbers is mounted under the riverbed at the upstream of bridge piers. When a flood comes, the riverbed elevation change is measured by considering the positions and numbers of the bricks swept away. The disadvantage of the method is that it requires digging in the riverbed for mounting the brick array and each set of array can be used only once. Moreover, only the scour depth is learned while the sediment deposition is unable to detect.
Sliding magnetic collar: a slide bar with a hollow design is disposed on the surface of a bridge pier and a collar sleeve moves along the slide bar. When the flood scours, the collar sleeve moves downward to stop at a specific position corresponding to the change of scour depth. A magnetic sensor disposed in the slide bar moves along with the collar sleeve and sends movement signals to externals by wires. The disadvantage of this method is that it only records the scour depth but the sediment deposition is unable to be detected.
Auto-lens monitoring: a hollow, transparent and waterproof tube is embedded into the riverbed at a bridge pier and fixed at the bridge pier. A photographic equipment for monitoring is disposed on the holder, which is moved vertically on the rail under the control of a stepper motor so as to monitor the sand scour around. When the surface rises due to deposition of sediment or falls due to scouring, a real-time image recognition system is used to detect the sand surface. Thus, the scour change caused by either deposition of sediment or scouring can be detected. The disadvantage of this method is the large power consumption. Moreover, the transmission quality is unstable, easy to be interfered by the motor due to the wireless data transmission. Under harsh environments, the interface between the water and the sand surface may be difficult to judge. Furthermore, the device needs to be embedded into the riverbed.
Multiple-lens monitoring: this method is similar to the above method. However, the auto-moving lens driven by the motor is replaced by a plurality of multiple lenses. Each lens is used to track the scour or deposition positions of the sand surface. This method not only improves the power consumption problem, but also provides better transmission. However, the interface between sand and water is still difficult to be defined under harsh environments. Moreover, this method needs to embed the device into the riverbed.
Gravity-type scour measuring device: the sand surface falls due to scouring and the detector also falls along with the sand surface due to gravity. The elevation decreasing is measured so as to obtain the scour depth. The shortcoming of the method is the embedding of the measuring device and only the scour is detected. When sediments deposit, it is unable to detect.
Transmitter of scour monitoring for soil layer: a plurality of transmitters is embedded in the soil layer. When the sand surface is scoured to a certain degree, the transmitters are moved or disturbed to send signals so as to obtain the scouring of the sand surface. The disadvantage of the method is that this method is only applied to detect scouring of the sand surface. As to the sediment being deposited on the riverbed, it just doesn't work. Moreover, this is a one-shot method. And the transmitters need to be embedded into the riverbed.
Optical sensor: a measuring system that detects by fiber bragg grating. Once optical fiber is attached with medium, the refractive index in the optical fiber changes and wavelength shift occurs. Thus the depth of the medium can be estimated. The disadvantage of the method is that the optical fiber requires contacting the water surface or sand surface. The optical fiber is fragile and is easy to get damaged. Moreover, this method needs to embed the system into the riverbed.
Probe monitoring system: the device is disposed on a bridge pier. A measuring rod is driven to contact the riverbed for measurement of the elevation. A pressure sensor is disposed on the bottom of the measuring rod. When the measuring rod detects the sand surface under the water and contacts the sand surface, the pressure sensor is pressed and the measuring rod stops moving immediately. The scouring depth or deposition height of the sand surface is obtained by calculation of the depth that the measuring rod moved downward. The disadvantage of this method is that the length of the measuring rod should be quite long if the water surface is much higher than the sand surface. Moreover, the device is easy to be destroyed by external objects such as flooding or rocks.
The monitor methods available now all have certain limitations and shortcomings in use. Thus how to monitor the riverbed scouring at bridge piers by a better method for extending the lifetime of the bridge has become an important issue now.
Thus there is a need to find out a new monitoring system for riverbed elevation monitoring at bridge piers that overcomes above shortcomings and monitors the riverbed elevation in a real-time manner.